Make a spot welder

For posts specifically relating to fusor design, construction, and operation.
User avatar
Steven Sesselmann
Posts: 2111
Joined: Thu Aug 11, 2005 1:50 am
Real name: Steven Sesselmann
Location: Sydney - Australia

Make a spot welder

Post by Steven Sesselmann » Wed Dec 31, 2008 6:36 am

Hi Guys,

Yesterday I made a spot welder, it only takes a couple of hours and works perfectly for welding thin parts, such as your grid wires etc..

Firstly let me credit RF technologies for giving me the idea to use a MOT and Gabriel Mendez for suggesting the drill press.

Take an old microwave oven transformer and carefully remove the secondary coil (the one with the thinner wire).

Find a thick insulated copper cable and wind it three times around the iron core, where the primary was. (an old start cable works well).

Fit a switch and a 10 amp fuse in front of the primary

Make a couple of copper electrodes and solder these to either end of the cable.

Make up some insulators for the copper electrodes, I used nylon, but wood is okay too, as we are only dealing with 3 volts.

Rig up some kind of press, or if you have a drill press, then you have all you need

Have fun sticking bits of metal together..

1.jpg (129.11 KiB) Viewed 2845 times
2.jpg (135.95 KiB) Viewed 2845 times - Gamma Spectrometry Systems - Various papers and patents on RG

Posts: 95
Joined: Tue Nov 11, 2008 4:34 pm
Real name:

Re: Make a spot welder

Post by gabrielArgentina » Wed Dec 31, 2008 4:53 pm

Very simple and nice work!! congratulations!!!.

Posts: 58
Joined: Mon Mar 03, 2003 7:06 pm
Real name:

Re: Make a spot welder

Post by ScottC » Wed Dec 31, 2008 7:09 pm

I made something similar a few years ago. You may have trouble with the copper electrodes sticking to what you're welding. I made mine from brass with a tungsten insert in the center that I could change out. It worked well with small SS wire also. I like the drill press idea, I'll have to set something up like that for my mill.

Nice work.


Posts: 250
Joined: Sat Jun 30, 2007 8:07 am
Real name:

Re: Make a spot welder

Post by MarkS » Wed Dec 31, 2008 11:15 pm

funny story... it is what we used to make our grid

happy new years!
0714082202.jpg (86.22 KiB) Viewed 2845 times

George Schmermund
Posts: 262
Joined: Wed Aug 16, 2006 12:51 am
Real name: George Schmermund
Location: Carlsbad, CA

Re: Make a spot welder

Post by George Schmermund » Wed Dec 31, 2008 11:23 pm

Anything obvious in high vacuum is probably wrong.

User avatar
Richard Hull
Posts: 12295
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Make a spot welder

Post by Richard Hull » Fri Jan 02, 2009 8:03 pm

Nice job! that sort of thing was what I used for my first spot welder, until I found the super Raytheon electron tube element spot welder at a hamfest for $2.00! It weighed about 60 lbs.

The Raytheon uses the classic variable joule setting heat control of a capacitive discharge type and uses large thyratron tube switching to pulse the internal transformer with 600 volts stored in large capacitors. About all this gives you over your operation is a highly accurate, repeatable energy delivery so that once you "dial in" of find "your heat" you can replicate every weld for thousands of welds.

The one thing I had to get was the proper electrodes. Those I obtained at a local welding shop. The pro's use a special high phosphorus copper. Pure copper will stick at certain energies with certain materials. The proper electrodes never, ever, stick.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Posts: 51
Joined: Wed Nov 19, 2008 3:31 am
Real name:

Re: Make a spot welder

Post by Eldarion » Tue Jan 06, 2009 1:36 pm

does anyone know how to build a TiG welder from similar stuff?

Posts: 587
Joined: Tue Jan 02, 2007 6:58 pm
Real name:

Re: Make a spot welder

Post by tligon » Thu Jan 08, 2009 4:25 pm

I agree with Richard ... slip on down to the local welding supplier and see what they have in the way of welding electrodes. You can also order them on-line, or buy the stock material from some place like McMaster Carr.

In my case, I bartered for a beat-up, broken-down, real spot welder with an 80 joule capacity from my former employer. They owed me for a consulting job and were about to go bankrupt, so I saved somebody the cost of disposing of the thing. The thyratron-based charging circuit was shot (Richard probably has a tube for it in the drawer somewhere). I simply bypassed the fancy part and adjusted the primary voltage using an autotransformer. The real key was the selection of electrodes I found in a box in its accessory drawer, the mechanism for pressing them together, and the ability to adjust the electrode force at which the unit fired.

I grew spoiled using it. I left it with EMC2 when I came home, and saw it last summer, still in use in Dr. Nebel's lab out in Santa Fe.

Dog and Pony 2 is seriously in need of a new outer grid, and is not making a good star mode due to the present mismatch of the inner and outer grids. I have not taken the time to build a good new spot welder, so have been putting off building the replacements. I'd like to try the tantalum wire Richard gave me for a new inner grid. I really like the drill press approach.

Posts: 587
Joined: Tue Jan 02, 2007 6:58 pm
Real name:

Re: Make a spot welder

Post by tligon » Tue Mar 02, 2010 2:02 am

Man, I was shopping e-bay last night and spotted a Weldmatic like the one I donated to EMC2, listed for about $1k.

I've been asked to drag Dog and Pony II to at least one SF convention this summer, and the grids are a wreck. It is time to get off my fanny and do something about it.

E-bay has a number of hand-held spot welders available, all grossly overpowered welding 0.020" wire, but evidently containing a beefy step-down transformer and with a set of arms and electrodes that can be adjusted in force, with a triggering system driven by a hand grip. The more advanced ones use cycle-counting to adjust energy delivered. I suspect the cheaper ones may just run on a pressure switch.

I expect even the smallest of these would vaporize grid wire, but I'm intrigued by the mechanism. Has anyone here tried building a capacitor discharge system for one of these? Or just running one on an autotransformer at really low voltage (I have a 20A Variac). I could get a photoflash cap capable of storing aroung 60 joules at 160 V for about $40, and discharge it thru the primary.

The going price for a Pitbull DN-100S is about $120, shipped. There's another popular model by Harbor Freight for a bit more (which probably means the Pitbull is lower quality than HF carries!).

User avatar
Doug Coulter
Posts: 1312
Joined: Sun May 27, 2007 7:18 pm
Real name: Doug Coulter
Location: Floyd, VA, USA

Re: Make a spot welder

Post by Doug Coulter » Tue Mar 02, 2010 2:37 am

Here's the one I built and use for grids and type C thermocouple wire (which is tungsten/rhenium).
I also have the harbor freight one, which works well with some custom electrode tips (to reach into funny places) on a large variac so it does skinny stuff.

But the cap discharge one is far more repeatable for tiny things, controlled jaw pressure, energy, etc.

This is a couple old computer caps (from the mainframe days), a variable volt regulator, and a big SCR from an old X ray machine.

Most should be obvious from the pic. I was planning to write it up someday....

Wire welder (MIG) tips work well for this and don't weld to the work. Special alloy used (just try to do anything with them other than grinding -- warned you ;~).

Buying a tap for the MIG tips is both easier and cheaper than making a big copper electrode set, they're cheap enough to grind them to custom shapes, and they work better.
Why guess when you can know? Measure!

Post Reply